membrane function - regulation of intracellular pH and Ca2+

what is the extracellular pH regulated by

lungs and kidney

uses HCO3-/CO2 buffering 

what is intracellular pH regulated by

combination of membrane transport proteins

what is the optimum range for [H+]

• 20nM → 100nM

what happens to cardiac myocytes if [H+] increases

• resting is 60nM

• increase to 100nM causes 50% dec in contractility

what affects intracellular pH

• metabolism

• ischaemia

• consequence of activity of membrane transport proteins

distributed at equilibrium

• if H+ is not actively transported

• H+ at equilibrium with the Vm

• same as reversal potential

what happens to H+ to maintain pH

• actively extruded

how is intracellular pH measured

• cells incubated with lipid permeable ester of indicator

• indicator ester diffuses into cell and hyrdrolysed to ionic form inside, trapped

• indicator is excited by bulb or laser

• fluorescence

• calibrate → intracellular pH

how do we find out mechanisms from intracellular pH

• measuring change in intracellular pH when the equilibrium is disturbed

what happens if a pulse of NH4+ is added

• H+ influx / HCO3- extrusion

• then adding acid causes H+ extrusion and HCO3- accumulation

what happens if EIPA is added to the NH4+ pulse experiment

• cell remains acidic

• EIPA is a sodium hydrogen ion exchanger inhibitor

acid extruders

• in response to acidification

• H+/Na+ exchanger

• HCO3- / Na+ co transporter (SLC4 family)

acid loaders

• in response to alkalisation

• Cl-/OH- exchanger

• HCO3-/Cl- exchanger 

why is intracellular conc of calcium extremely low

• phosphate ions

  • form insoluble precipitate with calcium

• extrusion across cell membrane

• sequestration into organelles

• (for signalling)

how is calcium extruded and sequestered

• 3Na+/Ca2+ exchanger

• Ca2+/2H+ PMCA

• 2H+/2Ca2+ SERCA

electrogenic

net movement of charge generating a small electrical current

• eg SERCA or SGLT1

signalling via ca2+

• calcium binds to voltage gated channels, or receptor operated channels such as NMDA

• then bind to ryanodine receptor or IP3 receptor

• calcium release

what physiological processes does calcium control

• exocytosis

• contraction

• enzyme activity 

• cell division

• fluid secretion

• cell death 

how does the same signal generate a different response

• temporal differences

• spatial differences - localisation lipid rafts